Drainage Board Crushing at BasementWall and Slab Joint

[RFreund]

I'm curious to know if there is a concern for drainage board crushing at the joint of a foundation wall and basement slab. This detail seems fairly reasonable as a way to mitigate water intrusion through basement walls, but many times the basement wall is relying on the concrete slab to resist lateral forces. Especially in old basements which seem to typically have these type of water intrusion issues. See sketch below:

EIT

http://www.HowToEngineer.com

 

[oldestguy]

bones 206: My bet is if you were doing the job and noted the tight contact between wall and slab that you would figure out a way to leave spaced areas intact to carry that load or an equal method with jacks, etc.

 

[bones206]

I’m sure you could come up with a bracing scheme but I doubt many contractors do that when installing these interior drain tiles. That’s why I asked the question because I was curious how that aspect is being handled.

 

[oldestguy]

Maybe the concern is not applicable to the usual job or we would hear more (any) about this here. When one takes into mind ALL the loads on the wall and see no movements, the SF has to be good in most cases. I have some problem worrying about changes in loads based on increased saturation and possible future swelling soils, etc. where there is an on going seepage problem at cases such as this. So far no bulged in or leaning walls, etc. as with swelling soils. In cold climates maybe winter frost action, but rare in a case like here.

 

[BridgeSmith]

Regarding 5,500 lbs/ft.... I think that might be a generous interpretation of that information.

It seems fairly straightforward to me - 11,000 psf / 12 in/ft * 6 in = 5,500 lbs/ft. If their data sheet is wrong, we have a 40' tall retaining wall that isn't going to have proper drainage. I feel fairly confident that if the manufacturer publishes a data sheet that states the compressive stress capacity, the capacity meets or exceeds that value.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

Why not have the drain board just below the slab? AS someone noted... drill holes through the wall, below the slab? or, why have it at all?

Dik

 

[XR250]

I don't get involved in these systems for all of the reasons listed in this thread.
Honestly, for what they charge for these around here, it would not be much more expensive to do it correctly from the outside.
As far as the slab bracing the bottom of the foundation wall - yea, in theory it is supposed to. However, I usually see a shrinkage gap there. I generally do not count on it in a design and use the full anticipated wall height - which is usually only 4" more than if the slab was bracing it.
Either way, basement walls seem to function way better than the numbers say they should.

 

[RFreund]

Thanks for all the responses. Pretty good discussion. I apologize for my poor efforts trying to find published compressive strengths.

Why not have the drain board just below the slab? AS someone noted... drill holes through the wall, below the slab? or, why have it at all?

I beleive the problem is that water leaches through the wall (typically a masonry or stone basement wall, old walls), then gets onto the slab on grade and causes a mess. The idea is just to direct water to a sump not to reduce water pressure or lateral pressure. I think if you just install the drain tile below the slab there is a potential for the water to still collect on to of the slab and not get through the joint between the slab and wall. I'm sure some water will get through the joint even if poured tight, but you can't guarantee all will make it through. If the foundation is in poor condition, then other reinforcement is needed.

If you are demolishing the edges of the basement slab, how can it continue to provide lateral support?

Pretty much what Oldestguy said... You could do it in some sort of underpinning type of pattern. Leave 4' installed, install 4' and keep leap frogging.

EIT

http://www.HowToEngineer.com

 

[BridgeSmith]

If the only purpose of this is to keep the water from running out onto the slab, and the plan is to remove the edge of the slab anyway, I suggest putting the drain under the slab, leaving periodic openings between the slab and the wall (more of an intermittent strip drain), and sloping the slab down at the wall. That way, whatever runs down the wall, stays at the wall and drains below the slab.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bones206]

You could do it in some sort of underpinning type of pattern. Leave 4' installed, install 4' and keep leap frogging.

In theory you could do that but I suspect the contractor would increase their estimate 4x.

I tend to agree that most basement walls don’t really need the slab as a lateral support. This was discussed a few months ago here: thread507-456923. FWIW my family home is currently under construction and the plain concrete basement walls have been backfilled without top or bottom support and is perfectly fine, even with saturated soil from rain + melting snow. That being said, I think older construction or CMU walls or areas with problem soils probably warrant more attention to loss of bottom support.

 

[r13]

I'll seal the slab edge to prevent incidental water/mud intrusion, then provide depressed/shallow trench along the edge to diver the seep through water towards sump pit, which should be connected to the newly installed drainage pipe.

You can also build a brick false wall up to the height of the water damaged surface for a nice finished looking. Just leave a small space for the trench.